What is your diagnosis?
The photomicrographs show granular, beaded, intratubular casts that are brick red on the trichrome stain and lightly pigmented on the H&E stain. This morphology, when accompanied by acute tubular injury, is classic for myoglobin cast nephropathy which can occur in the clinical setting of rhabdomyolysis. Of note, the morphology of myoglobin casts is quite variable and can range from granular and beaded as shown to obstructive and globular (not shown). Immunohistochemical staining for myoglobin (see photomicrograph below) is very helpful, especially in cases with unusual cast morphology. In our lab, hemoglobin is always performed as a panel with myoglobin as these casts can be indistinguishable by morphology alone.
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